On 11/28/17, from 2am to 6am PST, BitTitan will shut down all web traffic to Manage.BitTitan.com and MigrationWiz.com (MSPComplete and MigrationWiz). While there will be no access at this time, migrations in progress should not be disrupted.
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As described by the TechNet article
Understanding the Performance Impact of High Item Counts and Restricted Views, Microsoft states "in Exchange 2007, improvements in I/O, larger page size, and increased cache can help enable an increase in the recommended maximum item count. With properly architected hardware, an acceptable user experience can still be maintained with item counts as high as
20,000 items."

Note that enumeration is only slow for the folders that exceed the maximum item count guidelines, and does not affect folders that are smaller than these recommended sizes.